The invention relates to an optical device for eliminating the stray light, and particularly to an optical device diverting the original transmitting path of the stray light beam to improve the quality of the output image and the luminance projected on a screen.
FIG. 1 is a schematic diagram of an conventional optical system employing a xe2x80x9cdigital micromirror devicexe2x80x9d (DMD) or a xe2x80x9creflecting type liquid crystal on siliconxe2x80x9d (LCOS). FIG. 2 is an enlarged schematic diagram of a conventional chromatic halo generated by a stop of a projection lens with an aperture angle greater than 10 degree in coordination with the DMD chip with 10 degree turning angle. As shown in FIG. 1 and FIG. 2, the conventional optical system, using the DMD chip or the LCOS consists of a light source 10, a integration rod 12, a stop 20 of an illuminating system, a reflecting panel (DMD or LCOS) 30, a stop 40 of a projection lens, and a screen 50 for display.
By means of apparatus mentioned above, the light beam is transmitted from the light source 10 into the stop 20 of the illuminating system through the integration rod 12. Then, the light beam is transmitted into the stop 40 of the projection lens after being reflected form the reflecting panel 30 in order to project the image on the screen 50.
When the DMD chips with 10 degree turning angle is used for projection, the optical system only allows the light beam with an aperture angle of approximate 10 degree to enter the stop 40 of the projection lens for achieving the best luminance and contrast ratio of the image. And, that is due to the inherent characteristics of optics of the DMD 30 currently used. Despite of above mentioned, we could use a method to improve the luminance up to a certain level. Such method is implemented by using a stop of the projection lens with an aperture angle greater than 10 degree in coordination with the DMD chip 30 with 10 degree turning angle. However, on the other hand, the area A near the periphery of the stop of the projection lens with an aperture angle greater than 10 degree receives the additional light beam transmitted from the area B near the periphery of the stop 20 of the illuminating system. Such additional light beam is reflected from the DMD 30, and is transmitted into the stop 40 of the projection lens, and then is projected on the screen 50 that results in the chromatic halo 60. Such chromatic halo 60 always exists in any image so that it affects the quality of the output image. Especially when a black image is projected, since the existence of the chromatic halo results in an imperfect black image, the contrast ratio of the optical system is therefore affected. The chromatic halo 60 is resulted from the area A near the periphery of the stop of the projection lens with an aperture angle greater than 10 degree, and the area A is conjugated to the area B near the periphery of the stop 20 of the illuminating system. Thus, for any image being projected, the light beam at the area B is always projected by the projection lens through the DMD chip 30 (whether DMD is on or off) so as to generate the chromatic halo that causes flaws in the image projection. Furthermore, the second light source of point form at such area contributes a certain level in terms of the lumina output by the optical system. Therefore, if the light beam at this area is completely covered, the amount of lumina output by the optical system is greatly affected.
In view of the conventional flaws caused by the chromatic halo 60 in the image projection mentioned above, a small-sized reflecting mirror (with a flat surface or a curved surface) or a small-sized prism is provided at the area B according to the invention. Wherein, either a stop of a projection lens with an aperture angle greater than 10 degree is employed in coordination with a DMD chip 30 with 10 degree turning angle, or a stop of a projection lens with an aperture angle greater than 12 degree is employed in coordination with a DMD chip 30 with 12 degree turning angle. Thus, the light beam from the light source at this area is reflected by the reflecting mirror or is refracted by the prism so as to divert the original transmitting path of the light beam. Then, the light beam is reflected from the DMD 30 and is uniformly transmitted into the stop of the projection lens so as to increase the contrast ratio of the projection system and to improve the luminance output by the optical system. Such small-sized reflecting mirror or prism is installed near the periphery of the stop or the second light source of point form of the illuminating system so as to enhance the output lumina and to increase the contrast ratio of the optical system. The illuminating system, in which a stop of a projection lens with an aperture angle greater than 10 degree is used in coordination with a DMD chip 30 with 10 degree turning angle, or a stop of a projection lens with an aperture angle greater than 12 degree is used in coordination with a DMD chip 30 with 12 degree turning angle, and any design uses an aperture angle greater than that angle limited by the DMD chip, is implemented by the invention. Thereby, the luminance of the optical system is improved, and the best contrast ratio and the quality of image are achieved.
In order to achieve the object mentioned above, an optical device for eliminating the stray light (xe2x80x9cstray lightxe2x80x9d is the common name for above mentioned chromatic halo 60) is provided according to the invention. The optical device is provided at a stop 20 of a illuminating system in an optical system of the xe2x80x9cdigital light processingxe2x80x9d (DLP) that consists of the stop 20 of the illuminating system, a DMD 30, a stop 40 of a projection lens, and a screen 50. And, the stray light is the chromatic halo 60 at the peripheral area on the screen. Wherein, the optical device includes a reflecting mirror designed of an adjustable angle and installed at the area B near the periphery of the stop of the illuminating system. And, the chromatic halo generated by the light beam at the area B near the periphery of the stop of the illuminating system is reflected from the DMD and is transmitted into the stop of the projection lens, and then is projected on the screen 50. And, the reflecting mirror is used to reflect the light beam at the area B near the periphery of the stop of the illuminating system so as to divert the original transmitting path of the light beam. Thus, the light beam at area B is reflected from the DMD and is uniformly transmitted into the stop of the projection lens. Thereby, the contrast ratio of the optical system is increased, and the quality of the output image is improved, and the efficiency of the DLP system is enhanced, and the luminance projected on the screen 50 is improved.
These and other objects, features and advantages of the invention will become apparent with reference to the following description and accompanying drawings as follow.